Do I owe him an explanation for dumping him?

I met a guy on a dating app few months ago. It all went good, we were going on dates, we kissed, we had sex, in my opinion it was going with the flow. We talked about what we are looking for, I told him that for now I just want to date and to see where it goes, and that I never date multiply people in the same time and would like if he's the same, but if not that I would appreciate he would tell me. He told me he thinks the same. He then said he's not seeing anyone else and has no intention to. He also deleted his dating app. Fast forward few months, he got extremely busy with his work project (it's true he even showed it to me), so we weren't seeing each other that often anymore. He said he will be done with it in 2 months. I didn't mind, I'm also a busy person so I understood. We would still hang out few times a week and that was cool with me. But something just felt off. I don't know what, with no real proof I had a feeling he is hiding something. I've dealt with this gut feeling in the past and it never failed me. So I started snooping on his social media profiles (yes, call me a stalker) and what do you know - I found out he has A GIRLFRIEND. Maybe not the most exclusive relationship ever, but still a relationship. He hid it well on his social media, but when I found hers there it was, everything. Pictures, posts, statuses about two of them. And it started before we met.I am furious and hurt. And I don't even want to speak to him. I kinda know that stalking him was not something that I do and it's not right, so if I tell him what I found out he will call me a stalker and maybe even share it with other people. Right now I just want to ghost out on him, even though I know that's also not right. I just really really really have no energy for any sort of conversation and I don't want to risk getting into drama if she also gets involved. What should I do? Should I just make up some lie or ghost out? Or tell him the truth?

Updates:

I'm not sure if they are exclusive or not, but should I maybe tell her? Maybe she doesn't even know what he's doing, if it were me I would like to know. When we met he told me about her few times as "a friend of mine" so I didn't really thought much of it. But in her pictures they are kissing, hugging, laying in bed together, etc. so it doesn't look like just a friend to me. Her FB relationship status is "single" by the way, so that's also kinda confusing.

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Just tell him you're not feeling the relationship anymore and want to end. He doesn't have to know your exact reasons. That way he has a explanation to work with and you aren't ghosting. Just keep reminding yourself what he hid from you as a motivation. It might seem cold but he deserves it.

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What Girls Said 2

Tell him why you're dumping him, just say "because of your girlfriend" or something like that. No need to go in detail about how you found out. You can if he asks specifically. What he did is worse than what you did. He cheated, you just checked some social media.